"Treatment with aducanumab produced a dose- and time-dependent
reduction of amyloid plaque in the brain. Amyloid plaque is
believed to play a key role in the development of the symptoms of
AD (Alzheimer's disease)."

When the drug was tested at a high dose, it showed a 71%
reduction in the plaque after 54 weeks compared with the control
drug. Investors had been expecting a 30% reduction, according to
TheStreet's Adam Feuerstein. These percentages are measured
by the Clinical Dementia Rating used by the industry. The trial
was small and preliminary, leaving questions about whether it
will work in a larger study, according to
Forbes.

The price per share rose 6% to about $460 in early trade Friday.

And analysts think shares could power even higher.

In a note Friday, Barclays raised its price target to $500 from
$435, a 15% increase, and keeps an "Overweight" rating on the
stock.

Piper Jaffray also has a price target at $500 and also rates
Biogen shares "Overweight."

At $500 per share, the company would be worth over $117 billion;
its current market cap is $101 billion.

This portion of Piper Jaffray's note explains why biotech stocks
in general can be so eye-popping:

With millions of people in the US with Alzheimer's, these drugs
have the potential to become the biggest pharmacological class of
all time. The urgency to diagnose accurately and treat
should create a massive swell of demand for
any product deemed to have an attractive profile.

Essentially, investors take huge risks on biotech companies by
betting that their treatments would eventually create a drug that
cures a rare or terminal disease.

And the industry has been one of the best performing; it's up
nearly 16% year-to-date and over 32% in the past 12 months. The
S&P 500, in contrast, is up about 2% this year and 12% over
the past 12 months.

But these soaring prices are precisely what have some on Wall
Street worried that the sector is in a bubble. Even Federal
Reserve Board Chair Janet Yellen, in comments about stock
valuations to Congress last year, singled out biotech and
technology stock valuations as being "substantially
stretched." The industry has rallied by more than 50% since
then.

But the thing with bubbles is no one really knows until it pops.

if my point hasn't been made enough times, biotechs still don't care what yellen thinks